Interlaced scan vs. progressive scan image rendering

There are two basic ways in which video images can be displayed, or “rendered” on electronic display screens: interlaced scan and progressive scan. You have probably heard of these two methods of producing an image and may be wondering what the difference is between them, and how each of the technologies works.

Likewise, you have probably heard of the resolution standards 480i, 480p, 720p, 1080i and 1080p. The “i” and “p” after the number actually stands for “interlaced” and “progressive” respectively, and since these are the resolutions you will be choosing between when you are thinking of buying a new television system (especially an HD TV), it is important to know what the difference is between, say, a 1080i resolution TV and a 1080p resolution TV to make sure that you choose the kind that is best for you.

This Bamboo AV Advice article discusses interlaced scan and progressive scan and describes how each display method produces an image. Interlaced scan and progressive scan are then compared and the advantages and disadvantages of interlaced and progressive scan are summarised.

What is an interlaced scan signal?

An interlaced scan signal is way of displaying video and is a technique that has been used since the invention of the television, designed specifically for use with the old fashioned, large Cathode Ray Tube (CRT) televisions. Essentially, the interlace technique was invented to improve the picture quality of a video signal on a CRT device without consuming extra bandwidth. NTSC standard definition television broadcasts, VHS tapes and laserdisc signals are all encoded in the interlaced format. It is the way the majority of older televisions and some High Definition sets display an image.

How does an interlaced scan signal produce an image?

“Scanning” as a technology refers to how the image is “drawn” or “painted” on an electronic screen. The picture on the screen is made up of 480 horizontal lines, which is divided into two sets, odd numbers and even numbers. “Interlaced” means the lines that make up the picture on your TV screen are drawn in an alternating fashion. To display an image, the television sequentially draws all of the odd numbered lines from top to bottom and then proceeds to fill in the even lines. This occurs at a rate of 30 frames per second, meaning the entire process only takes around 1/30 of a second.

What is a progressive scan signal?

While this interlaced scanning worked well for older, analog televisions, it was not ideal for the new standard of electronic display devices that use a Liquid Crystal Display (LCD). A way to refresh the screen more frequently was desired. As a result, an alternative way of displaying images known as a progressive scan has been designed for those using modern devices such as Thin Film Transistor (TFT) monitors or digital cameras. Progressive scan means the lines that make up the TV picture are displayed all at once in sequence. Progressive scanning was originally called “sequential scanning” and is also referred to as “noninterlaced scanning”.

How does a progressive scan signal produce an image?

What differentiates a progressive scan from an interlaced scan is that the progressive scans system draws every line on the screen in just one pass, rather than in two passes. Each line is filled in sequential order instead of the alternating odd then even order that the interlaced scan system uses. This results in a faster refresh rate of once every 1/60th of a second.

Is interlaced scan or progressive scan more popular?

NTSC (National Television System Committee) and PAL (Phase Alternating Line) are the dominant analog video formats that are used throughout the world. While they differ in numerous ways, such as the amount of lines on screen (525 for NTSC, and 625 for PAL), they both use interlaced signals. This fact solidifies interlaced scans as the most popular technology of rendering images.

However, the only reason why progressive scan images were not originally used for TV signals is that they required too much bandwidth to send down analog lines. This has now changed thanks to the advent of digital TV (DTV) signals such as HDTV.

When buying a television, consumers are offered the choice between units that use progressive scans or interlaced scans, signified by the letter “p” or “i” at the end of the description, `080p or 1080i for example. Since televisions with progressive scan capabilities offer the viewer a better picture, albeit at extra expense, this type of signal is becoming more and more popular.

With regards to computers, progressive scan is most definitely the more popular method of rendering images. While the interlaced image rendering method was used in the 1980s, the 1990s saw a growing need for better resolutions. However, when the higher resolutions were implemented in conjunction with an interlaced monitor, large amounts of image flickering would occur. The eyestrain and distraction caused by this flickering of the image led the computing industry to abandon interlaced monitors almost immediately and switch too progressive scan image rendering instead.

What are the advantages of interlaced scan signals?

Reduced bandwidth

One of the most important factors in any video system is the bandwidth it uses, since more bandwidth means a more expensive and complex system. Interlaced video reduces the signal bandwidth by a factor of two, for a given line count and refresh rate when compared to progressive scan devices.

Popularity

Currently interlaced scan is the most popular format in which broadcasters output their TV signals. This is due to the reduced bandwidth that interlaced scanning requires.

Lower prices

The internal workings of televisions or other display units using the interlaced scan method of image rendering are far less complex than progressive scan devices, meaning that prices for interlaced units are generally much lower than progressive scan units

The disadvantages of interlaced scan signals

Image artefacts

During high-motion videos, images rendered by interlaced devices are prone to distracting image artefacts. This is because each frame of interlaced video is composed of two segments that are captured at different moments in time. If the recorded object – for example, a fast moving sports sequence, is moving fast enough to be in different positions when each individual segment is captured, a “motion artefact” will result.

Intentional image blurring

To counter the problem of image artefacts, images produced on interlaced scan systems are sometimes intentionally blurred, thus producing an image of lesser quality.

Improper reproduction of finer details

Image flickering

On larger screens particularly, an irritating flickering effect can sometimes become apparent. This flickering is also called “interline twitter” and is caused by the image on the screen containing vertical detail that approaches the horizontal resolution of the video format. You will have seen interline twitter in the past even if you didn’t realize this is what the phenomenon is called. Whenever you have seen a TV presenter’s striped shirt or suit flickering on a screen, this is interline twitter in action!

What are the advantages of progressive scan signals?

No image artefacts

None of the image artefacts associated with interlaced images are apparent in systems using progressive scan technology, because the lines on the screen are displayed sequentially, not at two different times.

No blurring

As a result of the lack of image artefacts, no intentional blurring is necessary in progressive scan systems.

Higher vertical resolution

In comparison to an interlaced image, progressive scan outputs an image of a higher vertical resolution.

Higher quality images

Images are smoother are more detailed in progressive scan as they are refreshed at a faster frequency.

The disadvantages of progressive scan signals

Higher bandwidth means lack of use

As an image using a progressive scan requires a higher bandwidth than an interlaced image of the same size, broadcasters using analogue signals hardly ever use progressive scan images. Only a minute amount of broadcasters using digital signals output content in 1080p, instead preferring to use 1080i.

It is fairly clear that should you have the budget, a television with progressive scan capabilities is always the preferable choice. It is especially designed for current High Definition sets and offers a far clearer, better quality image than an interlaced scan which was developed over fifty years ago with older CRT televisions in mind.